This invention relates to an improved system and method for mooring floating platforms, vessels, or other offshore equipment in deep and ultra deep water.
Currently, offshore oil and gas drilling and production can be done through the use of floating platforms and other vessels which are moored to the sea floor for stability. For those floating vessels working in relatively shallow water, a catenary mooring system is usually used. This system consists of a plurality of chain or wire moorings which are connected to anchors set into the sea bottom. In this mooring system, the chain or wire moorings used are not under a high degree of tension and sag under their own weight such that they do not form a straight line from the anchoring system to the floating platform or vessel. As a result, the platform is allowed a limited amount of movement, known as a xe2x80x9cwatch circle,xe2x80x9d caused by ocean currents.
At depths greater than 850 fathoms, the catenary system begins to be ineffective for drilling and production purposes. Because of the deep water, there is more slack in the mooring lines, resulting in a larger watch circle. Because it is important to limit the motion of floating platforms to within certain limits, especially in rough seas, the catenary system is not effective for use in deep and ultra deep water (greater than 2,000 feet).
In previous deep water mooring systems, buoys have been placed in the line of regular catenary mooring wires in order to keep these wires from drooping. These systems, however, were primarily used with wires or chains having less weight than those ordinarily used with conventional catenary mooring systems. The use of this system in deep water also resulted in an unacceptable watch circle during rough weather, especially in seas deeper than 2,000 feet. This resulted in a large amount of down time for the drilling or production operations due to bad weather.
Two additional mooring systems have been developed for use in deep and ultra deep water. These are the taut wire system and light wire catenary system. These systems, however, require major changes to both the floating platforms and mooring systems in order to stay within acceptable watch circle limits.
The taut wire mooring system is currently the primary mooring system used in deep and ultra deep water. This system consists of anchoring a floating platform directly to the sea floor by a plurality of mooring lines such as wires or cables. These mooring lines, however, must be under a very high degree of tension, such that they have no slack, in order to keep a floating platform within safe watch circle limits. In order to create this amount of tension in the mooring wires, a platform with an extremely high degree of buoyancy is necessary. One type of such a high buoyancy platform is described by Horton in U.S. Pat. No. 4,702,321. Normal platforms used in shallow water do not have sufficient buoyancy to be used effectively with the taut wire system.
The taut wire mooring system also cannot normally be used to moor vessels because of their limited size and buoyancy. Thus, only very large platforms can support the cable tension necessary for this type of system to be effective. This necessitates building large specialized platforms for use in deep water, resulting in higher production and drilling costs than those associated with conventional floating platforms having catenary mooring systems.
A second mooring system currently used with both platforms and vessels in deep and ultra deep water is a light wire catenary system. This system comprises the use of an ordinary catenary system with very light-weight mooring lines. These mooring lines are usually ropes made of a nelly buoyant, manmade fiber, such as KEVLAR. The ropes are then anchored to the sea bed as a regular catenary system. This system, however, allows a much greater watch circle than a taut wire system would in the same depth of water, resulting in a more unstable platform and increased down time in deeper water and in rough seas.
The size of the mooring ropes can also create additional problems. The mooring ropes necessary for use in this system are very bulky (approximately six inches in diameter). Because a vessel to be moored in deep water requires a very large amount of this rope, and because the rope is very bulky, most vessels do not have enough space to store the necessary amount of rope. Thus, such vessels need to be either modified and/or specially designed. Additionally, rope of this size is often too large for the winstrom used on ordinary vessels. Thus, this equipment needs to be specially designed as well.
Therefore, there is a need for a mooring system and method for use in deep and ultra deep water which does not require special equipment such as very large, ultra buoyant platforms, specially designed ropes, or special equipment to store and release these ropes. There is also a need for a deep water mooring system that can accommodate an unmodified shallow water platform or vessel. A mooring system is also needed that can use a conventional floating platform""s preexisting catenary mooring system in deep and ultra deep water while maintaining an acceptable watch circle.
The present invention is directed to an improved mooring system and method for deep and ultra deep water. This invention satisfies the needs described above by allowing shallow water vessels and platforms to be moored in deep and ultra deep water without requiring any modification to their conventional catenary mooring systems. This system also allows conventional floating platforms to be moored in deep and ultra deep water while maintaining an acceptable watch circle. This method works by, in effect, xe2x80x9cfoolingxe2x80x9d the platform into thing that it is in shallow water, or by xe2x80x9craisingxe2x80x9d the sea floor to a shallower level.
A version of this invention is comprised of a plurality of submerged buoys which are anchored to the sea bed by mooring lines at very high tension such that the buoys remain substantially stationary. Each buoy contains an anchoring means such as a hook, ring, or other means used to moor vessels, such that a shallow water floating platform can be anchored to these buoys by a regular catenary system. In this way, the buoys xe2x80x9cbecomexe2x80x9d the sea floor and the platform is fooled into thinking that it is in shallow water.
This system requires a plurality of submerged buoys (at least three) which are tethered at very high tension to the sea floor. The buoys are fixed at a substantially stable position at a predetermined depth by at least three mooring lines per buoy which are anchored to the sea floor. The more mooring lines between a buoy and the sea floor, the more stable that the buoy will remain. Also, the buoys"" depth should be determined by the normal working depth of the floating platform. Thus, if the maximum operating depth of a floating platform is 200 feet of water, the buoys should be placed at some depth less than 200 feet below the surface.
The number of buoys used will also affect the stability of the floating platform. At least three buoys must be used. However, the more buoys used, the smaller the watch circle of the floating platform will be.
By using this method, it is not necessary to utilize a specially designed floating platform or vessel in order to moor in ultra deep water. By xe2x80x9craising the sea bed,xe2x80x9d normal shallow water platforms can be moored and operated in deep and ultra deep water with no modifications, and while maintaining an acceptable watch circle.
These and other features of the present invention will be readily apparent from the following Description of the Drawings in which a preferred embodiment of the invention is shown.